Insulating films with low dielectric constant (low-k) are critically required for semiconductor manufacturing (see for example International Technology Roadmap for Semiconductors, Interconnect chapter, 2007 edition). Low-k films are usually created by introducing pores containing air or other gases, which have a dielectric constant close to 1, into a matrix material with dielectric constant in the range 2.0-3.0. The effective dielectric constant of the resulting porous film is typically 2.2 or less.
The low-k matrix material is typically a hydrogenated carbon-doped silicon oxide (SiCOH), wherein the free surfaces are terminated with methyl groups (CH3) bound to silicon. Processing steps, such as etching or chemical mechanical polishing, effectively remove the methyl terminations, leaving either dangling bonds or hydroxyl groups (Si—OH). As a result, the film becomes more hydrophilic and can readily absorb moisture. This in turn leads to an increase in the dielectric constant, the degree of which depends on the severity of the damaging process.
Another effect of carbon depletion is its impact on critical dimensions. For instance, the etching process used to form a trench through the low-k film would tend to leave the trench walls depleted of carbon. In subsequent wet stripping or cleaning processes the trench can be significantly broadened, a problem that becomes even more critical as feature size is reduced.
One solution to this problem is to repair the film by restoring carbon atoms with a silylating agent. Several families of compounds have been used as silylating agents for low-k repair, particularly alkoxysilanes, chlorosilanes, and aminosilanes. Of these, alkoxysilanes are the least reactive, but have the advantage that their chemistry is completely compatible with the SiCOH film. Chlorosilanes are difficult to handle due to their high reactivity with atmospheric moisture, as well as their tendency to form hydrochloric acid (HCl) as a silylation reaction byproduct. HCl may be problematic for metallic films present elsewhere in the circuit.
The use of aminosilanes for low-k repair has been demonstrated in several sources. In U.S. Pat. No. 6,395,651 to Smith et al. claim a nanoporous silica dielectric film treated by and a process for treating the same with a surface modification agent, such as hexamethyldisilazane (HMDS). The authors demonstrate that various exposure methods of exemplary films to HMDS resulted in a hydrophobic film surface, whereas an untreated film remained hydrophilic based on water droplet contact angle experiments. In a further example, Hacker et al. in U.S. Pat. No. 7,029,826 claim a method of imparting hydrophobic properties to a damaged silica dielectric film by contacting the damaged film with a surface modification agent, such as methyltriacetoxysilane (MTAS). U.S. Pat. No. 7,345,000 claims a method of treating a dielectric film by exposing the film to a treating compound and an alkyl silane, wherein the treating compound preferably includes HMDS, chlorotrimethylsilane (TMCS), trichloromethylsilane (TCMS), and combinations of these.
In U.S. Pat. No. 7,179,758, assigned to IBM (International Business Machines Corporation), compounds with two functional groups are considered as preferred over their mono-functional analogs because, in theory, a di-functional molecule could react with two neighboring Si—OH groups and mono-functional compounds can only react with one Si—OH. However, it is also suggested that a post-silylation annealing step is used to “condense” remaining Si—OH groups, allowing new Si—O—Si bonds to form. IBM's exemplary treatment processes suggest that bis(dimethylamino)dimethylsilane (BDMADMS), a di-functional molecule, provides superior hydrophobic properties compared to HMDS, a mono-functional molecule.
Considering the above references either separately or in conjunction, one can see that reaction compounds having at least one Si—N bond at the active site appear to be the most effective for surface modifications of silica-based films. This concept was concluded in the work of K. McMurtrey in the Journal of Liquid Chromotography, 11(16), 3375-3384 (1988), wherein several trimethylsilyl-(TMS) donors were compared in terms of their reactivity with silanol groups at the surface of silica gel particles. The results of this work suggest that trimethylsilyl-imidazole (TMSI), having a nitrogen-containing ring functional group, was more effective for this surface reaction compared to all of the above-mentioned compounds, as well as others known in the art such as trimethylsilyldimethylamine (TMSDMA). While the granted patents referenced above indeed claim TMSI as a treating compound for either surface modification or dielectric repair, the field of nitrogen-containing ring molecules remains unexplored in the art to date.